Intrathoracic impedance measurements can be performed by an implantable medical device (IMD), such as a pacemaker or implantable cardioverter defibrillator (ICD). Intrathoracic impedance measurements are typically made by measuring the impedance between two implanted cardiac electrodes or between a cardiac electrode and the IMD housing. The intrathoracic impedance measurement provides impedance across a portion of the thorax, which may include any portion of the heart, lungs and/or pulmonary vascular bed. Intrathoracic impedance decreases when thoracic fluid level increases, for example with pulmonary edema associated with worsening congestive heart failure. The impedance decreases because of the relatively low resistivity of biological fluids. As such, monitoring intrathoracic impedance is useful in monitoring the status of a heart failure condition.
Other changes that may cause an increase or decrease in intrathoracic impedance include ventricular dilation, vascular dilation, dehydration, hemorrhage, tamponade, pleural effusion, infection, electrolyte imbalance (e.g., hypercalcemia), and anemia. As such, a single intrathoracic impedance measurement can be useful in detecting a change in thoracic fluid status but may not be specific as to the location or underlying cause.